Phenol is an important product in the chemical industry and is useful in, for example, the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process via cumene. This is a three-step process in which the first step involves alkylation of benzene with propylene in the presence of an acidic catalyst to produce cumene. The second step is oxidation, preferably aerobic oxidation, of the cumene to the corresponding cumene hydroperoxide. The third step is the cleavage of the cumene hydroperoxide generally in the presence of a sulfuric acid catalyst into equimolar amounts of phenol and acetone, a co-product.
It is known that phenol and cyclohexanone can be co-produced by a variation of the Hock process in which cyclohexylbenzene is oxidized to obtain cyclohexylbenzene hydroperoxide and the hydroperoxide is decomposed in the presence of an acid catalyst to the desired phenol and cyclohexanone. Although various methods are available for the production of cyclohexylbenzene, a preferred route is disclosed in U.S. Pat. No. 6,037,513, which discloses that cyclohexylbenzene can be produced by contacting benzene with hydrogen in the presence of a bifunctional catalyst comprising a molecular sieve of the MCM-22 family and at least one hydrogenation metal selected from palladium, ruthenium, nickel, cobalt, and mixtures thereof. The '513 patent also discloses that the resultant cyclohexylbenzene can be oxidized to the corresponding hydroperoxide which is then decomposed to the desired phenol and cyclohexanone co-product.
There are, however, a number of problems associated with producing phenol via cyclohexylbenzene rather than the cumene-based Hock process. Firstly, oxidation of cyclohexylbenzene to cyclohexylbenzene hydroperoxide is much more difficult than oxidation of cumene and requires elevated temperatures and the use of a catalyst, generally a cyclic imide, such as N-hydroxyphthalimide (NHPI), to achieve acceptable rates of conversion. However, cyclic imide catalysts are expensive and, when used using to catalyze the oxidation of cyclohexylbenzene, the selectivity to cyclohexylbenzene hydroperoxide decreases with increasing conversion.
In the conventional cumene-based Hock process, the cleavage catalyst is normally sulfuric acid. However, even for the cleavage of cumene hydroperoxide, there are significant disadvantages of using sulfuric acid as the catalyst: 1) sulfuric acid is corrosive, especially in the presence of water, requiring expensive materials for reactor construction; 2) sulfuric acid needs to be neutralized before product separation and distillation, which requires additional chemicals such as phenate, caustics, or organic amines; and 3) the salt generated from neutralization requires separation and disposal and the waste water needs to be treated. Therefore, there are strong incentives to replace sulfuric acid with a heterogeneous cleavage catalyst that eliminates these drawbacks.
The patent and academic literature is replete with suggestions for replacing sulfuric acid in the cleavage of cumene hydroperoxide. Moreover, although less interest has been focused on the cleavage of cyclohexylbenzene hydroperoxide, International Patent Publication No. WO2011/001244 discloses that cyclohexylbenzene hydroperoxide can be converted to phenol and cyclohexanone in the presence of a variety of homogeneous or heterogeneous acid catalysts selected from Brønsted acids and Lewis acids. Suitable homogeneous catalysts are said to include protic acids selected from sulfuric acid, phosphoric acid, hydrochloric acid, and p-toluenesulfonic acid. Solid Brønsted acids such as those sold under the trade name Amberlyst™ and Lewis acids selected from ferric chloride, zinc chloride, and boron trifluoride are also disclosed. In addition, suitable heterogeneous acids are said to include zeolite beta, zeolite Y, zeolite X, ZSM-5, ZSM-12, and mordenite.
Further, Japan Unexamined Patent Publication 2007-099746 discloses that cycloalkyl benzene hydroperoxides can be cleaved with high selectivity to phenol and cycloalkanone in the presence of montmorillonite, silica-alumina, cationic ion exchange resins, sulfonic acid, perfluorosulfonic acid, and heteropolyacids supported on a carrier. Similarly, Japan Unexamined Patent Publication 2007-099745 discloses that cycloalkyl benzene hydroperoxides can be cleaved with high selectivity to phenol and cycloalkanone in the presence of aluminosilicate zeolites having pore diameter of 0.6 nm or greater, such as zeolite Y and zeolite beta.
However, the replacement of sulfuric acid with heterogeneous cleavage catalysts is not without its attendant problems. Thus, with heterogeneous catalysts, especially in fixed-bed operations, the cleavage feed is to be diluted with cleavage product and recycled, in order to better manage the reaction heat and to control reaction temperature. According to the present invention, it has now been found that, by diluting the cleavage feed with unreacted alkylaromatic compound (e.g., cyclohexylbenzene) from the oxidation step rather than, or in addition to the cleavage products, it is possible to achieve an advantageous integration of heterogeneous catalysis in the cleavage step with a cyclic imide catalyst in the oxidation step. In particular, the oxidation step can be deliberately operated at low conversion, thereby leading to shorter residence times in smaller oxidation reactors, lower levels of and better stability for imide catalyst, and higher selectivity to cyclohexylbenzene hydroperoxide. With this approach, the cyclohexylbenzene hydroperoxide in the cleavage feed is already sufficiently diluted with unreacted cyclohexylbenzene to provide the required heat management in the cleavage reaction. In addition, the low level of cyclohexylbenzene hydroperoxide in the cleavage feed gives the possibility of a single-pass, recycle-free cleavage operation, which can save both capital and operating costs.